The invention relates to the technical field of bonding by molecular adhesion of two substrates to one another.
Bonding is one way to attach two substrates to each other, which finds application, e.g., in the silicon-on-insulator fabrication technology called SMARTCUT®. Bonding by molecular adhesion is a technique during which two substrates are brought in close contact to each other and wherein the surface properties of the substrates are such that they stick to one another, without the application of further adhesives. The process of bonding is specifically started by applying pressure locally to the two substrates that are placed in close contact, from where a bonding front then spreads out over the entire interface of the two substrates.
WO 2007/060145 discloses such a process for bonding by molecular adhesion. The bonding method described therein comprises, prior to bonding, a step consisting of modifying the surface state of one and/or the other of the substrates to be able to regulate the propagation speed of the bonding front. The surface state is modified by locally or uniformly heating the surface of one and/or the other of the substrates to be bonded. The heating serves to dissolve water from the surface of the substrates before bonding, which allows minimizing of bonding defects. Bonding defects are, for example, so-called edge void defects, which result from the presence of water at the interface. In the SMARTCUT® process, this kind of defect can lead to the presence of non-transferred zones in the final product, e.g., an SOI wafer.
Nevertheless, even using the process of WO 2007/060145, it has been observed that, in a fabrication line, the number of defects rises with the number of substrates bonded inside the bonding tool. Accordingly, there is a need for improved processes that avoid this problem and these are now provided by the present invention.